Basal GABA regulates GABA_BR conformation and release probability at single hippocampal synapses

Presynaptic GABA_B receptor (GABA_BR) heterodimers are composed of GB_1a/GB₂ subunits and critically influence synaptic and cognitive functions. Here, we explored local GABA_BR activation by integrating optical tools for monitoring receptor conformation and synaptic vesicle release at individual presynaptic boutons of hippocampal neurons. Utilizing fluorescence resonance energy transfer (FRET) spectroscopy, we detected a wide range of FRET values for CFP/YFP-tagged GB_1a/GB₂ receptors that negatively correlated with release probabilities at single synapses. High FRET of GABA_BRs associated with low release probability. Notably, pharmacological manipulations that either reduced or increased basal receptor activation decreased intersynapse variability of GB_1a/GB₂ receptor conformation. Despite variability along axons, presynaptic GABA_BR tone was dendrite specific, having a greater impact on synapses at highly innervated proximal branches. Prolonged neuronal inactivity reduced basal receptor activation, leading to homeostatic augmentation of release probability. Our findings suggest that local variations in basal GABA concentration are a major determinant of GB_1a/GB₂ conformational variability, which contributes to heterogeneity of neurotransmitter release at hippocampal synapses.